Some types of mass spectrometers, for example time-of-flight mass spectrometers with orthogonal ion injection, require a very well-conditioned ion beam for high mass resolution and precise mass determination. By a “well-conditioned ion beam” we mean here a beam of ions flying as parallel as possible with kinetic energies which are as uniform as possible. This “ion beam conditioning” can consist in first decelerating the motion of the ions in a conditioning cell by numerous collisions with a collision gas, drawing the decelerated ions out of the conditioning cell through suitable diaphragm systems, and then forming them into a relatively fine, almost parallel ion beam. The process of reducing the kinetic energy of the ions by decelerating them in a collision gas is termed “thermalization”. This reduces the “phase volume” of the ions. By “phase space” we mean the six-dimensional space made up of space and momentum coordinates measured in an entrained system of coordinates; by “phase volume” we mean that part of the phase space which is filled with ions. Good beam conditioning always requires compression of the phase volume.
For a time-of-flight mass spectrometer with orthogonal ion injection, high mass resolution requires that a fine ion beam as parallel as possible with a diameter of only 0.5 millimeters if possible be generated, whereby the energy of the ions in the beam should be as uniform as possible, for example 20 electron volts with deviations of less than 0.5 electron volts. Ions from normal ion feeding systems, for example RF ion guidance systems, have a much larger phase volume and therefore have to be conditioned before being fed into a mass spectrometer of this type.
Conditioning the ions by reducing their phase volume like this cannot be achieved by ion-optical methods (a consequence of the Liouville theorem) and with the exception of the complicated method of laser cooling, only the gas cooling described can reduce the phase volume. U.S. Pat. No. 4,963,736 (D. J. Douglas and J. B. French) describes an RF-operated ion guidance system which conditions the ions by cooling them for optimum injection into a mass-selective quadrupole filter.
Ion storage cells filled with collision gas have proved successful in reducing the phase space, whereby the cells consist, for example, of four round rods positioned between the diaphragm systems on the input side and output side, and which use a supply with both phases of an RF voltage to build up an essentially quadrupole alternating field which, in conjunction with retaining potentials on the diaphragm systems, retains the ions in the storage cell.
The demands on the conditioning cells are particularly high if it is intended that the conditioning cells will also be used for the fragmentation of ions, i.e. when it is intended that the deceleration gas will be simultaneously also used as the collision gas for a fragmentation. For fragmentation, the ions are injected into the collision-gas filled system with kinetic energies of between 30 and 200 electron volts. The fragmentation process is denoted by the abbreviation CID (collisionally induced decomposition); the fragmentation occurs only after many collisions, when the ion has absorbed sufficient intrinsic energy as a result of the high proportion of collisions to lead to the fragmentation of a bond. Regardless of whether the ions are fragmented or not, they are also kinetically cooled in the collision gas simultaneously and in competition with the fragmentation, i.e. their kinetic energy decreases. The fragmentation process in these quadrupole systems would proceed more effectively in collision gases with a heavier molecular weight; these heavier gases cannot be used, however, since their gas molecules deflect the ions more strongly to the side during collisions and it is then very easy for the ions, as a result of such collision cascades, to escape laterally out of the round-rod quadrupole system.
All current tandem mass spectrometers require collision cells for the fragmentation of one species of ion (the “parent ions”) in order to obtain information about the structure of the parent ions by analyzing the fragment ion spectrum (or “daughter ion spectrum”). In general, the parent ions are selected from a primary ion mixture by a quadrupole filter; then fragmented in the collision cell; after fragmentation, the daughter ions can be analyzed in quadrupole mass spectrometers, time-of-flight mass spectrometers with orthogonal ion injection, in RF ion traps or in ion cyclotron resonance spectrometers.
For many years, RF quadrupole systems have been used as collision cells, which are usually constructed of round rods and operated with purely RF voltage without superimposed DC voltage (in the so-called “RF-only mode”), usually with helium as the collision gas (sometimes with nitrogen), and in which both the parent and also the daughter ions remain trapped as well as possible. Mass spectrometers which use quadrupole filters near the inputs and near the outputs to select or analyze ions have become known as “Triple-Quads”, for obvious reasons; these Triple-Quads have been known for around 15 years.
Collision cells usually consist of RF rod systems with round rods, although for high-quality quadrupole mass spectrometers, hyperbole systems, which permit significantly better separation efficiency and transmissions, have established themselves in the last 30 years. Inexpensive round-rod systems are still considered good enough for the collision chambers, expensive hyperbole systems are not used at all.
From the work of F. von Busch and W. Paul, Z. Phys. 164,588 (1961), however, it is already known that in round-rod quadrupole filters, non-linear resonances exist which lead to the ejection of those ions whose motion parameters lie in the middle of the “Mathieu stability zone” and which should therefore be collected in a stable state. In three-dimensional RF ion traps, these resonances lead to the phenomenon of “black holes”, which occur in the same way in rod systems, particularly in round-rod systems. Round-rod systems contain octopole and higher even-numbered multipole fields of considerable strength superimposed on the quadrupole field, leading to a distortion of the ion oscillations in the radial direction and hence to the formation of overtones of the ion oscillation. Their meeting with the Mathieu side bands leads to the resonances, which only occur, however, when the ions sweep through relatively wide radial oscillations. For ions lying damped in the axis of the system, the resonances are not effective. The Mathieu stability field is traversed by numerous non-linear resonance lines, the resonances are by no means rare.
Now it is precisely the case in collision cells that the ions injected with higher energies of between 30 and 200 electron volts must reach the vicinity of the rods or their intermediate areas in large numbers by means of collision cascades, and they are therefore inevitably subjected to the phenomenon of non-linear resonances if they fulfill the resonance conditions. Specific species of daughter ions can thus disappear from the collision cell and hence out of the daughter ion spectrum and thus adulterate the spectrum of the daughter ions. In the most unfavorable case, even the selected parent ions are subjected to this resonance and disappear to a large extent from the collision cell.
Apart from this, round-rod systems have the further disadvantage that the pseudopotential wall between the rods is extremely low (for commercially available systems only some ten to twenty volts) and can easily be overcome by ions with an energy of 50 electron volts, usually the minimum energy required for fragmentation processes, by means of a random laterally-deflecting collision cascade. This escape affects both parent and daughter ions. The higher the mass of the collision gas molecules, the more ions are lost, because in this case, the angles of deflection per collision are greater. A cascade of a few collisions which coincidentally deflect in the same lateral direction is enough to remove the ion from the collision cell. In the case of a very light collision gas, the larger angles of deflection of a small number of collisions are no longer able to compensate statistically as well as the large number of smaller angles of deflection.
As far as the conditioning of the ions is concerned, a disadvantage of most collision cells is that either the ions leave the cell again with relatively high energy after sweeping through once, since their energy has not been sufficiently reduced by collisions, or that, after a sufficiently large number of collisions (after a long sweep at high pressure or also after several sweeps with reflections at the ion output) they have given up their kinetic energy apart from residues of thermal energy and then remain in the collision cell. There has been a long search for collision cells which make it possible to construct an axial DC voltage drop to fish out the fragmented and thermalized ions from the collision cell in an efficient and uniform manner. The DC voltage drop needs only to be a few volts.
The easiest way to generate a DC voltage drop is in a quadrupole electrode system made of four thin resistance wires. The thin wires require an extremely high RF voltage, however, in order to build up the quadrupole RF field since the largest voltage drop occurs in the immediate vicinity of the thin wire. In addition, the resistance must not be particularly high, otherwise the RF alternating voltage cannot propagate along the wires sufficiently quickly. It is therefore only possible to generate very low DC voltage drops along the wire. Moreover, the pseudopotential wall between the wires is very low; the ions can escape very easily. Furthermore, the proportion of higher multipole fields is very high. Hyberbolic quadrupole systems comprising a large number of clamped parallel wires which imitate the four hyperbolic areas of the ideal quadrupole system provide a way out. Quadrupole systems replicated in wire like this were already being used around 40 years ago in the laboratories of Wolfgang Paul, the inventor of all quadrupole systems. These quadrupole systems are difficult to produce, however, and not very precise.
Another type of ion storage system with an electrically switched forward thrust is known from patent specification U.S. Pat. No. 5,572,035 (J. Franzen). The patent specification relates to various types of ion guidance systems which are completely different to the rod and wire systems described here. One of these consists of only two helical, coiled conductors in the shape of the double helix, operated by connection to the two phases of RF voltage. Another consists of coaxial rings connected in turn to the phases of RF alternating voltage. Both systems can be operated so that an axial forward thrust of the ions is generated. The double helix can be produced from resistance wire across which a DC voltage drop is generated, in a similar way to the quadrupole rod system made of thin wires; since the double helix is more compatible with thinner wires, however, and also has longer wires, it is more suitable for the DC voltage drop. The individual rings of the ring system can be supplied with a DC voltage potential which decreases in stages ring by ring, as also described in the patent.
Further solutions for collision cells which permit a thrust of the ions along the axis in the interior of the system are described in U.S. Pat. No. 5,847,386 (B. A. Thomson and C. L. Jolliffe) and protected by patent. All these systems are based on round rods:    (a) a segmented quadrupole system made of a chain of a few short rod systems whose potential on the axis falls off in stages;    (b) a quadrupole rod system made of conically tapering rods running parallel to the axis;    (c) a quadrupole rod system whose rods are arranged conically against each other;    (d) a quadrupole system of parallel rods with externally encompassing rings at DC voltage potentials which decrease step by step and which reach into the interior of the rod systemwhere they generate a decreasing potential on the axis;    (e) a quadrupole rod system whose nonconducting rods have an externally applied resistance layer across which a voltage drop is generated (better than the quadrupole system made of thin resistance wires);    (f) a quadrupole rod system made of insulating thin-walled ceramic tubes, with an external resistance layer for a DC voltage drop and an internal metal layer for the RF feed which acts through the insulator to the outside;    (g) a quadrupole rod system with auxiliary electrodes at weak DC voltage potential between the rods, whereby the auxiliary electrodes are arranged so as to taper to the axis of the system. The auxiliary electrodes are each located at the point of the zero potential of the two-phase RF voltage which is applied alternately across the rods. This generates a potential on the axis with potential gradient along the axis.
These arrays are, however, not completely satisfactory: partly because they are complicated to produce and therefore not particularly cheap, and partly because they function only moderately satisfactorily. The transitions between the split quadrupole systems thus present transmission losses and reflections in System (a). System (g) with the long auxiliary diaphragms between the rods exhibits larger ion losses in practice as a result of touching the auxiliary electrodes, which fundamentally decrease the height of the pseudopotential wall between the rods. This system has only limited suitability for the fragmentation of ions since the fragmentation always scatters the ions as well, and the losses are therefore much too high. The nonconducting rods (e) with resistance coating only partially conduct the RF voltage since here the higher capacity of the system compared with the thin wires means larger currents must be carried; or conversely, the resistance coating must really have an extremely low resistance. The ion guidance system (c), which is tapered instead of cylindrical, drives practically only those ions forward which have not collected at rest in the axis of the system, since only these experience a potential with a forward thrust. Almost the same is true for the rod system (b) comprising tapering rods. System (f) comprising thin ceramic tubes (according to the description tube walls around 0.5 to 1 millimeter thick) with interior metal coating to generate the RF field, and exterior resistance layer for the DC voltage drop, has disadvantages: the RF frequency causes such high dielectric losses in the material of the ceramic tubes that the system becomes extremely hot within a very short time and practically glows in the vacuum.
DE 102 21 468 A1 (J. Franzen and A. Brekenfeld) presents further systems with axial DC voltage drop, which are essentially based on the effect of DC voltages on externally encompassing tapering or trumpet-shaped electrodes.
It should be mentioned also that all rod systems into which external DC voltage potentials reach, as in U.S. Pat. No. 5,847,386, case (d) or (g), or as in DE 102 21 468 A1, are disadvantageous. The DC potential on the axis of the rod system is raised, thereby disturbing the parabolic minimum of the pseudopotential in the axis. In a quadrupole system of this type, four new potential minima in which the ions can oscillate are created between the axis and the rods. The possible oscillation amplitudes for the ions are extremely limited, however; the ions can easily collide with the rods and be lost through discharge.
The best systems are those which leave the parabolic minimum in the axis of the rod system undisturbed yet generate a DC voltage drop, as is the case with the rod system made of thin resistance wires or case (g) from U.S. Pat. No. 5,847,386, whose basic principle of the dielectric penetrated by RF has also been known for a long time. Every conductor radiates RF, whether it is insulated or not. A cylinder made of resistance material penetrated by RF has also been known as a “leaky dielectric” for some time (P. H. Dawson, “Performance of the Quadrupole Mass Filter with Separated RF and DC Fringing Fields”, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 375-392. Cited is: W. L. Fite, Rev. Sci. Instrum., 47 (1976) 326).
Multipole systems of a higher order can also be used as a collision cell. Such multipole systems comprise more than just two rod pairs. With more than two rod pairs, hexapole, octopole, decapole, dodecapole fields etc. are created. Both phases of a two-phase RF voltage are applied across two neighboring rods. Walls of a so-called pseudopotential then develop between the rods, as is the case with the quadrupole system, these walls hold the ions in the interior of the rod system. In contrast to the quadrupole system, the pseudopotential forms a flat trough in the vicinity of the axis in which the thermalized ions collect further away from the axis than is the case with the parabolic minimum of a quadrupole system. The more rod pairs there are, the shallower the trough. Multipole systems are therefore not as suitable as quadrupole systems for beam conditioning. In octopole systems, it is even possible to observe that the Coulombic repulsion of the ions causes them to collect around the fringes; the axis has a much lower ion density. For some types of mass spectrometer, the higher multipole systems cannot therefore be used as a collision cell for the analysis of the daughter ions owing to their poor beam conditioning.
Time-of-flight mass spectrometers with orthogonal injection of a primary ion beam possess a so-called pulser at the beginning of the flight path which accelerates a section of the primary ion beam, i.e. a string-shaped ion package, at right angles to the previous direction of the beam. This forms a ribbon-shaped secondary ion beam in which light ions fly quickly and heavier ones more slowly, and whose direction of flight lies between the previous direction of the primary ion beam and the direction of acceleration at right angles to this. A time-of-flight mass spectrometer of this type is preferably operated with a velocity-focusing reflector which reflects the whole width of the ribbon-shaped secondary ion beam and directs it towards a similarly extended detector.
If all ions fly in a line exactly in the axis of the pulser, and if the ions have no velocity components transverse to the primary ion beam, then theoretically—as can easily be understood—an infinitely high mass resolution power can be achieved, since all ions with the same mass fly precisely in the same front and reach the detector at precisely the same time. If the primary ion beam has a finite cross section, but no ion has a velocity component transverse to the direction of the beam, then spatial focusing of the pulser again theoretically means an infinitely high mass resolution can be achieved. The high mass resolution can even still be achieved if a strict correlation exists between the ion location (measured from the beam axis of the primary beam in the direction of the acceleration) and the ion transverse velocity in the primary beam in the direction of the acceleration. If no such correlation exists, however, i.e. if ion locations and ion transverse velocities are statistically distributed with no correlation between the two distributions, then it is no longer possible to achieve high mass resolution.
The primary ion beam must therefore be conditioned with respect to location and velocity distribution in order to achieve a high mass resolution in the time-of-flight mass spectrometer.
Beam conditioning is also required for other types of mass spectrometer, or at the very least it is useful. Every mass spectrometer has a phase space acceptance cross section which determines which of the injected ions are accepted and which deflected or reflected.